The field of alternative energy is rapidly growing. With more population and growth comes increased demand on power systems, causing increased consumption of fossil fuels and traditional energy sources. Delivering these energy sources from their source to recipients comes at considerable cost since large amounts of electricity are lost during delivery. These drawbacks to traditional energy sources have lead to increased interest in alternative energy sources.
One form of alternative energy is solar power. Making solar power cost-effective and reliable are two keys to bringing this alternative energy to the forefront of modern power systems. The cost to build and install solar panel systems has continued to drop, but not at a rate that would make solar panels a primary choice of energy for residential and commercial buildings. Solar panels also suffer from inherent negative characteristics that reduce the power output of the overall system. For example, when one portion of a solar panel system becomes shaded from the sun, it can reduce the power output of the entire system, even from panels that remain in full view of the sun. This same problem presents itself within a single solar panel—even partial shading of a single panel will reduce the output of the portions of a panel remaining in full sunlight. As a result, solar panel systems continue to be expensive and to provide an amount of power output that may fall short of fulfilling the power demands for a particular building or structure.
Common low-cost photovoltaic (PV) solar systems may utilize a centralized architecture, as illustrated in FIG. 1A. The output characteristic curve of the PV solar panel 102 or cell has a unique maximum power point (MPP) where the output power from PV panel 102 or cell is largest. Maximum Power Point Tracking (MPPT) controllers 104 may be used to ensure that PV solar panels 102 operate under their highest or close to highest power level under varying weather conditions. In this architecture, a single centralized power converter 106 with maximum power point tracking (MPPT) controller may be utilized to supply power to an off-grid or grid-connected load 108. Large numbers of PV panels 102 may be connected in series and in parallel to generate high voltage and high power/current. This type of centralized architecture finds its applications in PV solar power plants and in residential rooftop PV systems. The total output voltage (Vpv) and the total output current (Ipv) from the grouped PV panels are usually sensed to perform MPPT control. However, the disadvantage of the centralized architecture is that if the operating conditions of one or several panels are mismatched or subjected to partial shading conditions, the MPPT controller 104 is not able to guarantee the maximum power point of each panel, resulting in low maximum power point tracking efficiency. Due to the effects of mismatching conditions, instead of performing MPPT with one central power converter, a so called “string system” architecture, as illustrated in FIG. 1B, may be used. PV panels 102 are grouped into PV strings (several panels connected in series) and each PV string is connected to a power converter 110 which performs MPPT control for each individual string. The string voltage (Vst) and string current (Ist) from each PV panel string are usually sensed to perform MPPT control. The advantage of the string system architecture is that string converters perform MPPT at the string level. Therefore, the MPP tracking efficiency is higher than the centralized architecture under mismatching conditions between the strings. However, the string architecture solution still has significant amount of power losses under mismatching conditions within the same string. The reason is that, commonly, when PV panels 102 are connected into a string, a bypass diode can be placed in parallel with each PV panel. If a PV panel is shaded, this panel is bypassed in order to maintain the current level of the unshaded PV panels on the same string. Therefore, when PV panels operate under mismatching conditions, the power of the bypassed PV panels 102 is lost. If no bypass diodes are used across each panel, one shaded panel in a string will lead to significant power loss in that string because the shaded panel will limit the current in the string because of the in series connection.
To further improve the MPP tracking efficiency of the PV solar system under mismatched conditions and partial shading conditions, a module integrated converter (MIC) 112 PV solar system architecture may be used where each panel or group of cells has its own power converter and MPPT controller 104. FIG. 1C illustrates the parallel MIC PV solar system architecture and FIG. 1D illustrates the series MIC PV solar system architecture. The MIC architecture may allow MPPT to be performed for each individual PV panel, which alleviates the mismatching and partial shading conditions effects and therefore the MPP tracking efficiency is improved. In both series MIC architecture and parallel MIC architecture, the MPPT controller 104 requires sensing each PV panel's voltage (Vpv1, Vpv2, . . . Vpvi) and current (Ipv1, Ipv2, . . . , Ipvi) in order to perform distributed maximum power point tracking (DMPPT) for each individual PV panel. The tracking efficiency of this PV solar system architecture is higher than string and centralized PV solar system architectures. However, for an N-panel PV system, the number of required MPPT digital controllers 104 is N, the number of required analog-to-digital converter circuits is 2N and the number of required power converters is N, which results in significant cost increase.
Accordingly, despite multiple attempts to reduce the complexity of solar systems while increasing power output, significant drawbacks remain to existing systems. Another form of energy sources includes radio-frequency (RF) energy harvesting. RF waves are prevalent in modern technology and carry radio signals, cell phone signals, and television channels. These RF waves also provide a source of energy that can be converted to electricity.
With both of these and other forms of alternative energy sources, battery technology plays a central role in storing the energy until it is needed. Battery packs are made from individual cells that are electrically connected together. When one cell experiences problems, it degrades the performance of other cells also. In some systems, the performance of an entire battery pack may be limited to the performance of the worst cell. This means that when one cell goes bad, the entire battery pack becomes unusable. Cells are particularly likely to go bad where they are frequently charged and discharged, which happens often when batteries are used in alternative energy harvesting systems.
Taken together, there is a need for multiple improvements to modern energy harvesting systems. Solar panels can be reduced in cost, and complexity, while at the same time offering higher energy output. RF energy harvesting systems likewise can be made more efficient, and at reduced costs. And having better control over battery cells so that battery packs can continue in use even in the presence of isolated cell failures will aid with efficiently storing and dispensing stored energy.